Fixed TkellemServer#stop_bouncer

You missed one there :)

```
> network --remove --name=espernet
Error running command: undefined local variable or method `network_user' for #<Tkellem::TkellemServer:0x00000002c8ca10>
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_server.rb:99:in `stop_bouncer'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_server.rb:65:in `after_destroy'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_server.rb:135:in `block in after_destroy'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_server.rb:135:in `each'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_server.rb:135:in `after_destroy'
```